Various types of techniques that can be used to reduce a necessary force for operating steering apparatus have been known, among which is one where an assist mechanism is provided, between operating levers and operated members, for reducing the operating force to be applied by a human operator.
Assist mechanism of that type is proposed, for example, in JP-A-2-168067, according to which an operating-force reducing unit is provided between two pulleys operatively interconnected via a belt or cable. The following paragraphs describe the assist mechanism disclosed in the 2-168067 publication, with reference to FIG. 11 hereof.
The assist mechanism illustrated in FIG. 11 comprises an operating-lever-side pulley 203 that is rotated by operation of an operating lever 202, a controlled-object-side pulley 205 mounted to an object of control 204, and a power assist unit 206 disposed between these two pulleys 203 and 205.
The power assist unit 206 includes an input pulley 209 connected via an input-side cable 208 to the operating-lever-side pulley 203, an output pulley 212 connected via an output-side cable 211 to the controlled-object-side pulley 205, a strain absorber 213 that not only permits torque transmission between the pulleys 209 and 212 but also permits rotational flexure between the pulleys 209 and 212, and an electric motor 215 coupled via an electro-magnetic clutch 214 to the output pulley 212.
Further, in FIG. 11, reference numeral 216 indicates a controller for controlling the power assist unit 206, and 217 indicates a drive source.
As the operating-lever-side pulley 203 is rotated via the operating lever 202 to thereby rotate the input pulley 209, a direction and intensity of torque applied to the input pulley 209 are detected via a sensor 207. The detected signals are input to the controller 216. Once the torque intensity exceeds a predetermined level, the electromagnetic clutch 214 and electric motor 215 are turned on in response to an output signal from the controller 216, so that the output pulley 212 and electric motor 215 are interconnected and the electric motor 215 is caused to rotate. In this way, the output pulley 212 is given an assist force from the electric motor 215, which can thereby reduce the necessary force for manipulating the operating lever 202.
However, where the power assist unit 206 of the assist mechanism 201, including the input pulley 209, output pulley 212, strain absorber 213, electromagnetic clutch 214 and electric motor 215, is applied to a steering apparatus of a working machine, such as a crawler-type transport vehicle, it would present the problem that the construction of the apparatus tends to become large in scale.
Further, where the aforementioned assist mechanism 201 is to be employed in a steering apparatus of a working machine, a pair of left and right assist mechanisms 201, complicated in structure, are required, which would disadvantageously result in an increased cost of the working machine.
In view of the foregoing, it is currently desired to realize an assist mechanism of a simplified structure which is capable of effectively reducing a necessary force for manipulating a steering apparatus of a working machine, and to minimize the cost of the steering apparatus.